E46 Floor/Sub Frame query
Discussion
The problem with the E46 M3 floor and sub-frame are well known, and mine's gone in to Dick Lovett in Swindon for repair. Their body-shop manager told me that the problem's down to flexing caused by the power from the M3 engine. I'm seeing a friend on Friday who's got a B3 Alpina which has a little less BHP much the same torque as an M3, so should he be concerned and get his checked?
Its not unique to the M3 by any stretch, and from an engineering point of view, it is not the engine. The problem arises from the torque reaction of the differential which is restrained in the subframe. The subframe as a torsion member is very stiff, which in turn is bolted to a relatively compliant thin plate (the floor pan).
In a solid axle car, the diff is essentially restrained by the leaf springs. In a car with independent rear suspension, something has to react the force created at the pinion and crown gears in the diff. If these were not restrained, the gears would just spin off each other. In this case, the diff body has to react the separation force on the gears. This is then attached to the floor, and hey presto your forces are transferred to the floor. The problem is worse in lower gears as the torque is higher.
You are right in thinking the M3 may be worse, for a couple of reasons.
1. They tend to be driven harder
2. The torque from the engine is higher than other e46 petrols, albeit only at very high revs
3. The clutch is quite violent
4. The locking action of the diff adds some more complex transient loading to the diff body
In theory, high powered diesel e46s should be quite bad for the problem too. They can transmit more torque for a lower wheel speed in the same gear . Now I think in terms of fatigue; you are more likely to hit high/peaks torques in a diesel during daily driving than you are in an equivalent petrol as they occur so much lower down in the rev range.
In a solid axle car, the diff is essentially restrained by the leaf springs. In a car with independent rear suspension, something has to react the force created at the pinion and crown gears in the diff. If these were not restrained, the gears would just spin off each other. In this case, the diff body has to react the separation force on the gears. This is then attached to the floor, and hey presto your forces are transferred to the floor. The problem is worse in lower gears as the torque is higher.
You are right in thinking the M3 may be worse, for a couple of reasons.
1. They tend to be driven harder
2. The torque from the engine is higher than other e46 petrols, albeit only at very high revs
3. The clutch is quite violent
4. The locking action of the diff adds some more complex transient loading to the diff body
In theory, high powered diesel e46s should be quite bad for the problem too. They can transmit more torque for a lower wheel speed in the same gear . Now I think in terms of fatigue; you are more likely to hit high/peaks torques in a diesel during daily driving than you are in an equivalent petrol as they occur so much lower down in the rev range.
There are a few vids on Youtube (by Redish Motorsport I'm sure) that suggest the OEM subframe bushes are part of the issue. They have 2 raised sections where they meet the floor instead of being an even height all round, so in theory the load is transferred straight from these 2 points which press against the spot welds in that area and start to weaken/crack it.
I can imagine the SMG II cars can't help matters either, the shift in S5/S6 is bloody violent and like you say, they're not a car that will have been driven gently by previous owners.
I can imagine the SMG II cars can't help matters either, the shift in S5/S6 is bloody violent and like you say, they're not a car that will have been driven gently by previous owners.
The bushes...I've seen the redish opinion on them and its a fair point. The spot welds really are in an awful position to make the problem worse. But those nibs on the bushes...there must be a genuine reason for them rather than a flat abutment. They have the same nibs on the rear trailing arm bushes too. Maybe they're is an engineering function behind them, maybe its a manufacturing function. Who knows!
Al 450 said:
Strange one this as when I asked my BMW indy he said that he'd heard of the issue but had never seen an example of it on a car. He's a well regarded independent specialist who has E46's in every day.
It's not common in the great scheme of things, a few hundred posts on here over the years is nothing compared to the thousands of E46's on the road. Many of the main dealers I spoke to had done a handful over the years. The internet is great for blowing things out of proportion Gassing Station | M Power | Top of Page | What's New | My Stuff